The present invention relates generally to a cryogenic cooling system for synchronous machine having a rotor with a high temperature superconducting (HTS) coil. More particularly, the present invention relates to an evaporative cooling system to provide cryogenic fluid to the rotor and to re-cool used cooling fluid returned from the rotor.
High temperature super-conducting generators require highly reliable, low cost cryorefrigeration equipment in order to be viable as commercial products. To achieve high reliability with existing cryorefrigeration equipment involves redundant cryorefrigerator components. The inadequate reliability of these components and the requirement that HTS rotors have an uninterrupted supply of cooling fluid necessitates that redundant components be included in cryorefrigeration systems for HTS rotors.
However, the cost of cryorefrigeration systems is substantially increased due to the need for redundant cryorefrigerator components. Moreover, existing cryorefrigeration systems require frequent maintenance due to their inadequate reliability and system redundancies. Accordingly, the operating cost of these systems is relatively high.
The purchase and operating costs of existing cryorefrigeration systems significantly adds to the cost of machines having HTS rotors. These high costs have contributed to the heretofore commercial impracticalities of incorporating HTS rotors into commercially marketable synchronous machines. Accordingly, there is a substantial and previously unmet need for cryorefrigeration systems that are less expensive, inexpensive to operate and provide a reliable supply of cryogenic cooling fluid to a HTS rotor.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron forging.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher, which increase the power density of the electrical machine and result in significant reduction in weight and size. Air-core super-conductive rotors, however require large amounts of super-conducting wire, which adds to the number of coils required, the complexity of the coil supports, and the cost.
Super-conductive rotors have their super-conducting coils cooled by liquid helium, with the used helium being returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room- temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power. Accordingly, there is a need for a cryogenic cooling system that reliquefies the hot, used cooling fluid returned from the rotor. The reliquefied cooling fluid should then be available for reuse as a HTS rotor cooling fluid.
A highly reliable cryogenic cooling system has been developed for a HTS rotor for a synchronous machine. The cooling system provides a steady supply of cooling fluid to an HTS rotor. Moreover, the cooling system is economical in its construction and operation. The reliability and economy of the cooling system facilitates the development of a commercially viable synchronous machine with a HTS rotor.
The cryogenic cooling system is a gravity fed close-loop evaporative cooling system for high temperature super-conducting (HTS) rotor. The system comprises an elevated cryogen storage tank, vacuum jacketed transfer lines that supply liquid cryogen to the rotor and return vapor to the storage tank, and a cryorefrigerator in the vapor space of the storage tank that recondenses the vapor. A cryogenic valve controls the rate of gravity fed cryogen flowing from the storage tank to the HTD rotor. The cryorefrigerator may be a single stage Gifford-McMahon cryocooler or pulse tube with separate or integral compressor. The cryogenic fluid may be neon, hydrogen or other such cooling fluid.
In a first embodiment, the invention is a cooling fluid system for providing cryogenic cooling fluid to a high temperature super-conducting rotor comprising: a cryogen storage tank storing a liquid cryogenic cooling fluid; an inlet transfer line connecting the storage tank to the rotor and forming a passage for liquid cooling fluid to pass from the tank to the rotor; a valve connected to the inlet transfer line, wherein the storage tank is elevated above the rotor and the liquid cooling fluid is gravity fed to the rotor at a flow rate controlled by the valve.
In another embodiment, the invention is a cooling fluid system coupled to a high temperature super-conducting rotor for a synchronous machine and a source of cryogenic cooling fluid comprising: a cryogenic storage tank and a supply of cryogenic cooling fluid stored in the tank, wherein the tank is elevated above the rotor; an inlet line providing a fluid passage for the cooling fluid between the tank an the rotor; a valve coupled to said inlet line and regulating a flow of cooling fluid through the inlet line to the rotor; a return line providing a fluid passage for the cooling fluid between the rotor and tank, and a cryorefrigerator cooling the fluid in the storage tank.
In a further embodiment, the invention is a method for cooling a super-conducting field winding coil in a rotor of a synchronous machine using an elevated cryogen storage device and a valve in a line between the storage device and machine, wherein the method comprises: storing cryogenic cooling fluid in the tank, wherein the tank is elevated above the rotor; allowing the cooling fluid to flow under the force of gravity from the tank to the rotor; regulating the flow of cooling fluid to the rotor by the valve; cooling the field winding coil with the cooling fluid, and returning the cooling fluid to the tank.